1. Field of the Invention
The present invention generally relates to apparatus and methods for buoyancy compensation. More particularly, the present invention is directed to a selectively deformable buoyancy device and methods for its use.
2. Description of the Prior Art
Manned and unmanned vehicles used underwater often operate in or near the condition of neutral buoyancy. This condition allows precise, stable control of depth with minimum energy expenditures, especially when working in confined areas. Buoyancy is the upward force on any submerged body which is equal to the weight of water that would otherwise occupy the same volume. Since most components of a subsea vehicle weigh more than their volume of water, additional buoyant volume must be added to the vehicle to achieve a net neutral buoyancy.
Static buoyant volume includes hollow pressure vessels or syntactic foam, and cannot be altered during a dive. In contrast, dynamic buoyancy does allow some means to change the net displacement during a dive. It is chiefly used when vehicles are subject--by accident or design--to varying weights, volumes, or water densities in the midst of their dive. Most variance typically comes from changes to the content or position of onboard equipment caused by the execution of mission activities. However, all vehicles possess a degree of buoyancy instability because all materials compress and expand to some extent in response to changes of pressure and depth. This may also require small adjustments to displacement. A third use for dynamic buoyancy is adjusting the amount of traction a crawling vehicle can apply to the sea floor. Dynamic buoyancy can also be used as the propulsive force in an underwater glider.
In the first three applications, similar adjustments may be made with thrusters or lift from flow over control surfaces. But the latter requires a high velocity relative to the surrounding water which is impractical in many situations. And both require expending additional thruster power, which amount may be excessive or cause an undesired disturbance. Typical dynamic buoyancy systems are floodable "ballast" tanks that can be variably emptied or filled of the surrounding water by some combination of pumps and/or high pressure air. Alternatives include handling systems for oil and expendable weights. This equipment can be bulky and heavy in itself, and the effectiveness of gas-based systems drops off rapidly with the higher pressures experienced at more than 100 meters depth below the sea surface. Furthermore, such equipment can be difficult to miniaturize. A dynamic buoyancy system may in itself be heavier than the water and require a dedicated volume of state buoyancy to cancel most of its weight. Accelerations and attitude changes by the vehicle can move the parcels of water and air in the "ballast" tanks, creating fluctuating loads that impair vehicle control.
A typical dynamic buoyancy system expends pressurized gas every time it executes a full cycle of decreased and increased buoyancy. This limits the number of buoyancy adjustments the vehicle may make, requiring it to periodically cease productive operations and recharge the system from some source. A vehicle utilizing buoyancy as a sole source of depth control expends gas constantly as a result of the instability described above.
A dynamic buoyancy system may instead use pumps to reversibly transfer air between buoyancy and storage chambers. Such a system expends much less air at the cost of added weight, complexity, and noise.
The various disadvantages of contemporary buoyancy control systems may especially impact underwater robotic craft. Since these vehicles are often utilized and valued for the ability to perform dives of long duration, dynamic buoyancy systems that require frequent recharging are not desirable. Since these vehicles are typically smaller than manned submarines and submersibles, they are more sensitive to the size and weight of the complex handling equipment required to not expend buoyant material. Greater volume and weight make a vehicle less manageable and less propulsively efficient. Unmanned or robotic craft may also require a buoyancy system insensitive to extreme attitudes and accelerations.